The present disclosure relates to a reading module used in a digital copier, an image scanner, or the like, which irradiates a document with light and reads image light reflected from the document, and to an image reading device including the reading module, as well as to an image forming apparatus.
Conventionally, there are two types of reading methods for an image reading device mounted in a multifunction peripheral or the like utilizing electrophotographic process. One is a CCD method using a charge coupled device (CCD) sensor, and the other is a CIS method using a photoelectric conversion element called a complementary MOS (CMOS) sensor.
The CCD method is a method of reading an image that is reduced using an image sensor having a size that is ⅕ to 1/9 of a document size, and a plurality of flat mirrors and optical lenses. A merit of the CCD method is that a depth of field is large. The depth of field means a range in an optical axis direction in which the image looks in focus when an object (i.e. the document) is moved from the just focused position in the optical axis direction. In other words, it means that when a depth of field is large, not so blurred image can be read even if the document is shifted from a normal position.
On the other hand, a demerit of the CCD method is that it has a very long optical path length (light propagation length from the object to the sensor) of 200 to 500 mm. In order to secure the long optical path length in a limited space inside the carriage, the image reading device uses a plurality of flat mirrors to change light propagation direction. Therefore, the number of components is increased, and hence cost is increased. In addition, in a case where a lens is used in the optical system, chromatic aberration occurs due to a difference in refractive index depending on wavelength. In order to correct the chromatic aberration, a plurality of lenses are necessary. This also causes an increase in cost.
The CIS method is a method in which a plurality of erecting same-scale rod lenses are arranged in an array, and the image to be read is formed on an image sensor having substantially the same size as a document. A merit of the CIS method is that it has an optical path length of 10 to 20 mm, which is shorter than that of the CCD method, and has a small size. In addition, because only the rod lenses are used for imaging, mirrors are not necessary unlike the CCD method so that a scanner unit including a CIS sensor can be slimed, and hence cost can be reduced because its structure is simple. On the other hand, because the CIS method has a very small depth of field, if a document is shifted from a normal position in the optical axis direction, a variation in magnification among the lenses largely affects and causes image blur. As a result, the CIS method has a demerit that a book document or a non-flat document cannot be read uniformly.
In recent years, another method different from the CCD method and the CIS method described above is proposed. In this method, an image is read by using a reflection mirror array as an imaging optical system. This method uses a plurality of reflection mirrors arranged in an array so that read document images of reading areas corresponding to the reflection mirrors are formed on the sensor as a reduced and inverted images. However, unlike the CIS method using the rod lens array, one area is read and imaged by one optical system. In addition, a telecentric optical system is adopted as an imaging system, and hence image blur due to overlapping of images having different magnifications does not occur when a plurality of areas of a document are read separately. Thus, image blur is prevented, and a compound-eye reading method is realized.
Further, because this method uses only mirrors in the optical system, chromatic aberration does not occur unlike the case of using a lens in the optical system. Therefore it is not necessary to correct chromatic aberration, and hence the number of elements forming the optical system can be reduced.